A well-known method of detecting motion in a video signal utilizes a frame store for storing the video signal of an entire picture frame. The video signal that contains the picture information of a subsequent picture frame is compared with the stored video signal. The result of such comparison provides a motion signal. This is so because motion causes the content of a pixel in the incoming video signal to be different from the corresponding pixel in the already stored frame. A disadvantage of using this method, however, is that a costly frame store is required.
One example in which the motion detector of the invention may be used is a television apparatus for displaying the picture information of a television signal that was produced by an interlaced scanning process, hereinafter referred to as the interlaced signal, in a non-interlaced scanning format. The motion detector of the invention is particularly useful when such television apparatus utilizes a line store memory for adapting the interlaced signal for display in the non-interlaced scanning format. The adaptation to non-interlaced scanning format is aimed at reducing certain artifacts that are caused by displaying a picture using the interlaced scanning format at the television receiver, as described below.
In broadcast systems such as the 525 line-per-frame, 30 frame-per-second 525/30 NTSC system or the 625/25 PAL system, artifacts occur because of the interlaced scanning process. This process divides the 525-line picture or frame of the NTSC standard into two successive 2621/2-line fields. The 2621/2 lines of one field are scanned in 1/60th of a second followed by scanning of an additional 2621/2 lines of another field with lines of the second field occupying the spaces between the lines of the first field. One subjective effect of this interlaced scan is to create in the presence of motion in the picture scene an apparent vertical drift of the lines of the raster known as "line crawl". The apparent drift is substantially more noticeable when viewing a wide-screen display at close range. Another well-known objectionable visible effect is interline flicker on line-to-line transitions that occurs because of the interlaced scanning format.
Recent interest in the development of high definition television systems (HDTV) has been directed towards techniques that are intended to enhance the subjective performance of present systems within the constraints of existing standards. One approach, a technique referred to as progressive scan, or non-interlaced scan, has been described in patent materials and literature. For example, all the scan lines are scanned consecutively from the top of the display screen and downward to the bottom of the display screen during each vertical scanning interval of, for example, 1/60th of a second. Progressive scan results in the reduction of artifacts related to the interlaced scanning format, such as interline flicker and line crawl that exist in conventional two-to-one interlaced displays. The subjective effect is a more pleasing picture presentation to the viewer.
In U.S. patent application Ser. No. 526,702, filed Aug. 26, 1983, entitled, PROGRESSIVE SCAN TELEVISION SYSTEM EMPLOYING VERTICAL DETAIL ENHANCEMENT, in the name of D. H. Pritchard and W. E. Sepp (which issued Dec. 10, 1985, as U.S. Pat. No. 4,558,347), an apparatus for displaying the picture information of, for example, a baseband NTSC signal is disclosed. The television apparatus that is disclosed in the Pritchard et al. application generates, during each horizontal video line time of the NTSC signal, a pair of luminance information containing signals. The picture information that is produced in one of the pair of signals, during each horizontal video line time of the NTSC signal, is displayed in a first scan line of the display and that of the other is displayed immediately afterward in the next adjacent scan line below the first scan line. In order to display the luminance information that is included in both signals during one horizontal scan line time of the NTSC signal, each one is speeded up, or compressed. Thus, in the television apparatus of Pritchard et al., picture information is displayed in a given pair of scan lines during the same period in which picture information, in the standard interlaced television receiver, is displayed in only one scan line.
Each signal of the pair of luminance signals is generated using a line comb filter. Because of the interpolation or the averaging process that occurs in the summation process of the line comb filter, some loss of vertical detail occurs. Such loss of vertical detail may be noticeable in the section of the display where picture vertical transitions occur. The Pritchard et al. application discloses an approach for reinserting vertical detail signal components to compensate for the loss that occurs as a result of the averaging process. There, the vertical detail is obtained from the subtraction process in a comb filter source and reinserted in a specific manner to enhance the subjective vertical sharpness and to produce a "clean" subjective picture.
The non-interlaced scanning format reduces interline flicker and line crawl type artifacts that arise from the interlaced scanning format in standard television displays. However, artifacts caused by the interlaced scanning process at the source of the interlaced signal, such as at the television camera, are not eliminated by the non-interlaced scanning format of the television receiver. Furthermore, when vertical detail enhancement process is employed in the television receiver, such as disclosed in the Pritchard et al. application, the visibility of such source related artifacts may even be increased. Such artifacts are caused at the source of the interlaced television signal by the aliasing components that are included in the interlaced NTSC signal. The aliasing components in the interlaced NTSC signal are generated as a consequence of the overlapping of the field-to-field sidebands generated by the sampling process that is inherent in producing interlaced signals at the camera.
Because of the interlaced scanning format that is used at the source, the frequency spectrum of the signal that is produced by the television camera includes integer multiples of the horizontal scanning frequency of illustratively 15734 Hz with sidebands, of illustratively multiples of 30 Hz, the vertical rate, above and below each multiple of the horizontal scanning frequency. In processing a still image at the camera, the sidebands produced include frequencies that are spaced from the horizontal scanning frequency by integer multiples of 30 Hz. Because of the shape of the scanning spot at the camera, these sidebands may include aliasing components. These aliasing components are visible when the vertical components of moving objects are displayed in the television receiver display, and also when differences in the scene content due to television camera vertical panning occur from one field time to the immediately following one. Such differences in the scene are generally referred to as motion in the picture scene. When motion occurs, it may be desirable to omit the vertical detail enhancement to reduce the visibility of the artifacts that may be caused by the camera aliasing edge components.